Numerous poorly water-soluble bioactive substances, e.g. drugs, are present as solid, in particular crystalline bulk materials at room temperature, primarily in the form of poorly wettable powders with grain sizes in the micro- and millimeter size range. In many cases drugs which share these properties exhibit a poor bioavailability, particularly upon peroral administration. Bioavailability is defined as the rate and the extent of absorption of a bioactive agent into the blood compartment and of the distribution to its site of action. The low absorption rate of poorly water-soluble, in particular lipophilic substances from the gastrointestinal tract (GIT) is generally attributed to the poor solubility of these substances and to their poor wettability in gastrointestinal fluids. Industrially manufactured bioactive substances have generally particle sizes well above 1 .mu.m since they are preferably processed from cruder materials by mechanical comminution such as milling and micronization. In some cases precipitation from organic solvents is applied. Sjostrom et al. (Sjostrom B., Kronberg B., Carlfors J., J. Pharm. Sci. 82 (1993) 579-583) describe the manufacturing of submicron drug particles by precipitation in solvent containing o/w emulsions. The method is based on the use of potentially harmful organic solvents such as toluene and chlorinated hydrocarbons. From the technical point, it is virtually impossible to completely remove the solvents from the product so that the solid drug particles contain solvent residues which present a toxicological risk. Moreover, the use of volatile and inflammable organic solvents requires special precautions with respect to manufacturing safety.
Direct injection into the bloodstream (in an aqueous vehicle) is not possible with many drugs due to the poor aqueous solubility of these substances. The size of suspended, poorly water-soluble drug particles/aggregates is generally too large for intravenous administration because it exceeds the diameter of the smallest blood capillaries and would thus lead to capillary blockage and embolism.
In case of extravasal administration of solid drugs with the objective of a systemic drug action, the dissolution process of the substance can become the rate limiting step in absorption and might thus lead to a poor bioavailability. It is common knowledge that the dissolution rate of a substance is affected inter alia by its particle size, its wettability and with crystalline substances also by the energy required to overcome lattice forces. It can therefore be deduced that the bioavailability of poorly water-soluble bioactive agents can in principle be enhanced by the following three technological manipulations:
reduction of particle size, PA1 hydrophilization of particle surfaces to improve the wettability in aqueous media, and PA1 reduction of the crystallinity of the substance. PA1 1. The poorly water-soluble substance, e.g. ubidecarenone, or a mixture of poorly water-soluble substances is melted. Optionally, one or more additives which decrease the melting point of the poorly water-soluble substance(s) and/or impede or inhibit the recrystallization of the molten poorly water-soluble substance(s) can be added to the poorly water-soluble substance or mixture of poorly water-soluble substances. PA1 2. Optionally, one or more stabilizing agents (e.g. amphiphilic substances, surfactants, emulsifiers) are dissolved or dispersed in the melt or in the dispersion medium depending on their physicochemical characteristics. Stabilizers can also be added or exchanged after homogenization, e.g. by adsorption of polymers or by dialysis of water-soluble stabilizers. PA1 3. Preferably, the dispersion medium is heated to approximately the temperature of the melt prior to mixing and may contain e.g. stabilizers, isotonicity agents, buffering agents, cryoprotectants and/or preservatives. PA1 4. Optionally, the dispersion medium and the melt are added and predispersed to give a crude dispersion, for example by shaking, stirring, sonication or vortexing. Predispersing is preferably carried out at temperatures above the melting point of the substance or the mixture of substances or the mixture of substances and additives, e.g. stabilizers, respectively. Predispersing can he omitted for well dispersible systems. PA1 5. The (predispersed) melt is emulsified in the dispersion medium, preferably at temperatures above the melting point of the substance or the mixture of substances or the mixture of substances and additives, e.g,. stabilizers, respectively. Emulsification is preferably carried out by high pressure homogenization or by sonication, but may be also possible by high speed stirring, vortexing and vigorous hand shaking. The way of homogenization determines the particle size distribution and the mean particle size of PSMs. PA1 6. Bioactive agents which are to be incorporated into PSMs such as lipophilic drugs can be melted together with the poorly-water soluble substance(s) constituting the particles; be dissolved, solubilized or dispersed in the melt prior to emulsification; be incorporated in PSMs after homogenization, e.g. by sonication; or be adsorbed to the surface of the particles. The way of incorporation depends on the physicochemical properties of the bioactive agents to be incorporated. PA1 1. Since the bioactive substance formulated as PSMs is completely or partly present in an amorphous physical state, preferably in the liquid state, e.g. a supercooled melt, dissolution of the substance does not require or, respectively, requires less energy than the crystalline substance which needs to overcome lattice forces. The dissolution rate of PSMs is therefore increased as well as its solubility resulting in an enhanced bioavailability compared to the crystalline bulk substance. PA1 2 Disadvantages of carrier systems for bioactive substances such as low pay load of the carrier or toxicological side effects which are related to the carrier particles themselves can be circumvented by the use of PSMs. PA1 3. The .small particle size of PSMs in the nanometer size range can generally not be achieved by conventional comminution techniques such as milling, grinding or micronization. The small particle size results in a tremendous increase of the specific surface area compared to conventional administration systems such as (micronized) powders or granulates. Since the solubility is related to particle size via the dissolution rate, size reduction results in an increased dissolution rate. It is well known that the peroral bioavailability of bioactive agents depends on their dissolution rate in gastrointestinal fluids. Consequently, the bioavailability of bioactive agents formulated as colloidal PSMs can be improved. PA1 4. Formulation of PSMs according to the present invention with particle sizes in the nanometer size range renders possible the direct parenteral administration of practically water-insoluble substances. Due to the small particle size of PSMs, dispersions thereof can be administered intravenously without risk of embolism which is not possible for the crystalline bulk substance suspended in an aqueous medium. PA1 5. The achievable particle size of PSMs is below 150 nm which corresponds to the diameter of the fenestrations of the endothelial wall of blood vessels. Intravenously administered PSMs therefore have the potential to leave the vascular compartment via these fenestrations. Drugs formulated as PSMs or incorporated into PSMs can thus be transported within the particles to extravascular targets such as the bone marrow or tumor tissues. PA1 6. Since PSMs are covered by stabilizing agents, they have hydrophilic surfaces and therefore exhibit a good wettability. A good wettability, e.g. in the gastro-intestinal tract, facilitates the dissolution of the substance which leads to an improved bioavailability. PA1 7. The surface characteristics of PSMs can be modified e.g. by the choice of stabilizing agents, by the adsorption of polymers or by the attachment of so-called homing-devices such as monoclonal antibodies or carbohydrate moieties. It is possible to modify the bloavailability and the biodistribution with respect to the rate and extent of absorption, the circulation time in the vascular compartment, the transport to the site of action and the organ distribution by modification of surface properties. PA1 8. PSMs, in particular those formulated from pharmacologically inactive substances, can be used as (colloidal) carrier systems for poorly water-soluble, especially lipophilic bioactive agents such as drugs. Drug-loaded PSMs can be administered by enteral, parenteral, peroral, oral, mucosal, rectal, pulmonal, ophthalmic and (trans)dernal routes. PA1 9. The process of manufacturing of PSMs involves unexpensive conventional techniques only and provides a product which is sale with respect to its handling. In contrast to the production of extremely fine powders there is no risk of dust explosions, cross-contamination in a factory environment or inhalation of bioactive substances by personnel since the particles are present in an aqueous dispersion. Moreover, the production method of PSMs does not involve toxicologically hazardous additives such as chlorinated hydrocarbons or other organic solvents, and can be accomplished with physiological additives only thereby circumventing problems of toxic residues.
For example, improvement of the bioavailability after peroral administration due to enhancement of the rate of dissolution by micronization has been described for digoxin (Shaw, T. R. D., Carless. J. E., Europ. J. Clin. Pharmacol., 7 (1974) 269) und griseofulvin (Atkinson, R. M., Bedford, C., Child, K. J., Tomich, E. G., Nature 193 (1962) 588). Micronization is the comminution of agglomerates to microcrystals of a size between 1 and 30 .mu.m by means of appropriate comminution equipment such as vibration mills, fluid-energy mills and colloid mills. Micronized substances can, however, exhibit wettability problems, e.g. due to aerophilization during the milling process. The reduced wettability counteracts to the increased dissolution rate achievable by micronization as a result of the reduced particle size and can therefore lead to a reduced dissolution rate.
A further reduction from the micrometer to the nanometer size range, e.g. in order to further enhance the bioavailability or to render possible parenteral, in particular intravenous administration, is practically not feasible with the conventional equipment used for micronization or requires a tremendous technological effort, and is therefore extremely costly and in many cases ineffective. Additionally, the reduction of solids to submicron-sized powders can bring about heavy difficulties in handling of these dry products such as an increased risk of dust explosions and cross-contamination problems in a factory environment. Moreover, such systems present a risk to health for persons exposed to the possible inhalation and absorption of potent bioactive materials.
For many applications there is, however, an obvious need to reduce the particle size down to the nanometer range. Thus particle size is an important factor with respect to the parenteral, in particular intravenous administration of drugs. As already mentioned before, many lipophilic drugs can not be formulated as aqueous solutions due to their low aqueous solubility. Intravenous administration of suspensions to sparingly soluble substances in water bears the risk of capillary blockage and embolism since the suspended particles are generally larger than the smallest blood vessels.
So far there are basically only two possible ways of intravenously administering such lipophilic drugs. One possibility is to solubilize the drug in an aqueous medium by use of solubilizing agents such as surfactants and organic solvents. Although the use of these agents may increase the solubility of lipophilic substances to such an extent that therapeutic doses can be achieved, these systems have some considerable disadvantages. Intravenous administration of organic or alcoholic solutions is often associated with pain and local thrombophiebetis at the injection site. The use of high surfactant concentrations, which often are necessary for solubilization, can cause anaphylactoid reactions including anaphylactic shock, and is thus not advisable.
The second possibility is incorporation of poorly water soluble substances into colloidal drug carrier systems. Colloidal carrier systems comprise e.g. polymeric (micro- and) nanoparticles, liposomes, lipid emulsions and lipid suspensions. These drug carriers are vehicles of predominantly colloidal size, i.e. in the nanometer size range, in which the drug is incorporated. Due to their surface characteristics these vehicles can be dispersed in an aqueous medium. Since their size is--with the exception of microparticles--below 1 .mu.m, they are suited for intravenous administration.
Drug carrier systems in the micrometer size range are represented by microspheres consisting of a solid polymer matrix, and microcapsules in which a liquid or a solid phase is surrounded and encapsulated by a polymer film. Nanoparticles consist, like microspheres, of a solid polymer matrix, however their mean particle size lies in the nanometer range. Both micro- and nanoparticles are generally prepared either by emulsion polymerization or by solvent evaporation techniques. Due to these production methods, micro- and nanoparticles bear the risk of residual contaminations from the production process like organic solvents such as chlorinated hydrocarbons, as well as toxic monomers, surfactants and cross-linking agents which can lead to toxicological problems. Moreover, some polymeric materials such as polylactic acid and polylactic-glycolic acid degrade very slowly in vivo so that multiple administration could lead to polymer accumulation associated with adverse side effects. Other polymers such as polyaikylcyanoacrylates release toxic formaldehyde on degradation in the body. Furthermore, microparticulate carriers are not suited for intravenous administration due to their size in the micrometer range.
Drug carrier systems for parenteral administration which are based on lipids are liposomes submicron o/w emulsions and lipid suspensions. These systems consist of physiological components only thus reducing toxicological problems associated with the carrier.
Liposomes are spherical colloidal structures in which an internal aqueous phase is surrounded by one or more phospholipid bilayers. The potential use of liposomes as drug delivery systems has been disclosed inter alia in the U.S. Pat. Nos. 3,993,754 (issued Nov. 23, 1976 to Rahmann and Cerny), 4,235,871 (issued on Nov. 25, 1980 to Papahadjopoulos and Szoka), and 4,356,167 (issued Oct. 26, 1982 to L. Kelly). The major drawbacks of conventional liposomes are their instability on storage, the low reproducibility of manufacture, the low entrapment efficiency and the leakage of drugs.
Lipid emulsions for parenteral nutrition are oil-in-water (o/w) emulsions of submicron-sized droplets of vegetable oils such as soya oil or of medium chain triglycerides dispersed in an aqueous medium. The liquid oil droplets are stabilized by an interfacial film of emulsifiers, predominantly phospholipids. Typical formulations are disclosed in the Jap. Pat. No. 55,476/79 issued on May 7, 1979 to Okamota, Tsuda and Yokoyama. The preparation of a drug containing lipid emulsion is described in WO 91/02517 issued on Mar. 7, 1991 to Davis and Washington. Due to the high diffusivity of incorporated drugs in the oil phase, the drug is released relatively fast from the emulsion vehicle upon administration into the blood stream. The oil is degraded to untoxic metabolites by the body within several hours. There is, however, a certain susceptibility of these lipid emulsions towards the incorporation of drugs due to the mobility of drug molecules within the internal oil phase since diffusing molecules can protrude into the emulsifier film. This might cause instabilities which lead to coalescence. Moreover, the solubility of poorly water soluble drugs in vegetable oils is often also relatively low. These carrier systems can therefore be used as a drug delivery system only in a very limited number of cases.
There are a number of micro- and nanoparticulate carrier systems which can be characterized as lipid suspensions. In these carrier systems a solid lipid phase is dispersed as micro- or nanoparticles in an aqueous medium. So-called lipospheres disclosed by Domb and Maniar (U.S. Pat. No. 435,546 lodged Nov. 13, 1989; Int. Appl. No. PCT/US90/06519 filed Nov. 8, 1990) are described as suspensions of solid, water-insoluble microspheres formed of a solid hydrophobic core surrounded by a phospholipid layer. Lipospheres are claimed to provide for the sustained release of entrapped substances which is controlled by the phospholipid layer. They can be prepared by a melt or by a solvent technique, the latter creating toxicological problems in case the solvent is not completely removed.
A slow release composition of fat or wax and a biologically active protein, peptide or polypeptide suitable for parenteral administration to animals is disclosed in U.S. Pat. No. 895,608 lodged Aug. 11, 1986 to Staber, Fishbein and Cady (EP-A-0 257 368). The systems are prepared by spray drying and consist of spherical particles in the micrometer size range up to 1,000 microns so that intravenous administration is not possible. The latter also applies to wax microparticles described by Bodmeier et al. (Bodmeier R., Wang J., Bhagwatwar, J. Microencapsulation 9 (1992) 89-98), or to ibuprofen containing microspheres of cetostearic alcohol reported by Wong et al. (Won, L. P., Gilligan C. A., Li Wan Po A., lnt. J. Pharm. 83 (1992) 95-114). Both systems can be prepared by crude dispersion of the molten lipid using a high speed stirrer.
In an attempt to improve the intestinal absorption of lipophilic drugs, Eldem et al. (Eldem T., Speiser P., Hincal A., Pharm. Res. 8 (1991) 47-54) prepared lipid micropellets by spray-drying and spray-congealing processes. The micropellets are described as solid. spherical particles with smooth surfaces. The lipids are present in the crystalline state. Due to the particle size in the micrometer range these micropellets cannot be used for intravenous administration.
Lipid pellets in the nanometer size range for peroral administration of poorly bioavailable drugs are disclosed in EP 0 167 825 issued Aug. 8, 1990 to P. Speiser. The nanopellets represent drug loaded fat particles solid at room temperature which are small enough to be persorbed. Persorption is the transport of intact particles through the intestinal mucosa into the lymph and blood compartment. Solid lipid nanospheres for parenteral administration are disclosed by Muller and Lucks in DE 41 31 562. These solid lipid nanospheres have been reported to be crystalline (Weyhers H., Mehnert W., Muller R. H., Europ. J. Pharm. Biopharm. 40 Suppl. (1994) 15S). The crystalline state of lipid nanoparticles described by Westesen et al. could be demonstrated by X-ray measurements (Westesen K., Siekmann B., Koch M. H. J., Int. J. Pharm. 93 (193) 189-199). All these systems represent carrier systems. The matrix materials of these carrier particles are exclusively composed of non-bioactive, pharmaceutically inert lipids.
Beside applicability by the parenteral route, particle size is also an important parameter governing the activity of the reticuloendothelial system (RES). Upon intravenous administration colloidal particles are in general rapidly removed from the blood stream by cells of the RES such as phagocytic macrophages. The rate of blood clearance by the RES depends inter alia on the size of the colloidal particles. Larger particles are generally cleared more rapidly than smaller ones so that the latter have a longer circulation time in blood and thereby a higher probability of the incorporated drug to reach its target site.
A colloidal particle can leave the blood by basically two different ways. On the one hand, this is possible by (receptor-mediated) uptake into cells by way of phagocytosis or pinocytosis. These processes are similar to the uptake by RES macrophages. On the other, the particles can leave the vascular system by so-called fenestrations of the endothelial wall. These fenestrations can be found e.g. in liver, spleen and bone marrow, but also at sites of inflammation or in tumour tissues. The diameters of these fenestrac are up to 150 nm. Extravasation through these fenestrations is of importance with respect to drug targeting to extravascular sites.
Beside the particle size effect, the rate of RES uptake is inter alia governed by the surface characteristics of the colloidal particles such as surface charge and surface hydrophilicity. It is generally accepted that colloidal particles should he uncharged and hydrophilic in order to avoid RES uptake. Thus there is a possibility to divert colloidal particles away from the RES by modifying their surface characteristics, e.g. by coating with polymers (Troster S. D., Muller U., Kreuter J., Int. J. Pharm. 61 (1990) 85).
Surface properties play also an important role with regard to the dissolution process, e.g. in the GIT after peroral administration of poorly water soluble substances, and are therefore related to the bioavailability. Since apolar surfaces are only poorly wetted in aqueous media, another approach to increase the dissolution rate of sparingly water-soluble substances is thus hydrophilization of particle surfaces. From the field of pharmaceutical technology it is known that suitable surfactants are added to milled. hydrophobic powders as wetting agents in order to increase the wettability. Hydrophilization of apolar surfaces of poorly water-soluble bioactive agents can be obtained inter alia by processing these substances with water-soluble additives such as polyvinylpyrrolidone or polyethyleneglycol into spray-embeddings or co-precipitates.
Apart from reduction of particle size and improvement of wettability, the peroral bioavailability of a poorly water-soluble drug can be enhanced if the drug is not present in a crystalline but in an amorphous physical state. In general, amorphous forms of a substance exhibit a higher solubility and a faster dissolution than their crystal forms since the dissolution of amorphous substances does not require lattice energy. It is known, for example, that the antibiotic agent novobiocin can only be absorbed from the intestinum after administration of the amorphous substance which has a solubility ten times higher than the crystalline agent (Mullins J. D., Macek T. J., J. Am. Pharm. Assoc., Sci. Ed. 49 (1960) 245).
Ubidecarenone (6-Decaprenyl-2,3-dimethoxy-5-methyl-1,4-benzoquinone: coenzyme Q.sub.10) is an endogenous quinone. In the human body it is localized in mitochondrial membranes and is involved in electron transport in the respiratory chain. The reduced form of the molecule is an antioxidant. Ubidecarenone is therapeutically used inter alia in cardiomyopathia, coronary diseases and for the prophylaxis of heart attack (Folkers K., Lituuru G. P., Yamagami T., (Eds.), Biomedical and Clinical Aspects of Coenzyme Q. Vol. 6. Elsevier 1991). Due to its general cell protective and energetic properties the substance is also applied as nutrient supplement for daily preventive intake.
Ubidecarenone is crystalline at room temperature and has a melting point of 49.degree. C. (Rainasarma T., Adv. Lipid Res. 6 (1968) 107). The substance is commercially available as an orange-coloured powder consisting of crystal agglomerates in the micrometer to millimeter size range. Due to its long isoprenoid side chain the molecule is extremely lipophilic and practically insoluble in water.
The bloavailability of ubidecarenone is generally low due to the poor solubility in gastrointestinal fluids causing a low gastrointestinal absorption of the substance. Kishi et al. (in Folkers K., Yamamura Y., (Eds.). Biomediacal and Clinical Aspects of Coenzyme Q, Vol. 4, Elsevier 1984, pp. 131-142) observed that the peroral bioavailability of ubidlecarenone from solid dosage forms such as tablets and granules is related to the dissolution rate of the preparations. Kanamori et al. (Yakuzaigaku 45 (1985) 119-126) also report that the bioavailability of perorally administered ubidecarenone depends on the dosage form and decreases in the order soft gelatin capsule, granules and tablets.
A number of different formulations with the object to enhance the bioavailability of ubidecarenone can be found in the patent literature. Taki and Takahira disclose in EP 23349 (04.02.81) that the lymphatic absorption of orally administered ubidecarenone is increased by coadministration of long-chain fatty acids and monoglycerides. Increase of intestinal absorption by administration of capsules containing oily (surfactant) solutions of ubidecarenone is disclosed in different patents such as WO 8604503 A1 (14.08.86), JP 63188623 A2 (04.08.88), JP 62067019 A2 (26.03.87), JP 59148735 A2 (25.08.84) and JP 56012309 (06.02.81). Solubilization of ubidecarenone in micellar solutions is described in EP 522433 A1 (13.01.93), WO 8803019 A1 (05.05.88) and JP 59148718 A2 (25.08.84). Ueno et al. (Acta Pharm. Nord., 1 (1989) 99-104) report on the increase of peroral bioavailability by inclusion of ubidecarenone in a complex with .beta.-cyclodextrins. A similar formulation is disclosed in JP 56109590 A2 (31.08.81). Moreover, incorporation of ubidlecarenone in emulsions is reported to enhance intestinal absorption as described, for example, by Yano et al. in EP 494654 A2 (15.07.92).
For parenteral, in particular intravenous administration ubidecarenone has to be incorporated into a carrier vehicle since it is not possible to manufacture an aqueous solution with therapeutic concentrations of ubidecarenone due to its lipophilicity. Lecithin stabilized soya oil emulsions for intravenous administration of ubidecarenone are disclosed by Groke and Polzer (DE 3524788 A1, 22.01.87). Sugio et al. (JP 62123113 A2. 04.06.87) as well as Mizushima et al. (JP 60199814 A2. 09.10.85). JP 63319046 A2 (27.12.88) describes a soya oil emulsion vehicle coated by polysaccharides. The concentrations of ubidecarenone which can be incorporated in emulsions are, however, limited due to the relatively poor solubility of ubidecarenone in vegetable oils.
Liposome preparations of egg lecithin and cholesterol containing ubidecarenone are disclosed in EP 69399 A2 (12.01.83). Polysaccharide-modified liposomes are described e.g. in EP 94692 A1 (23.11.83). JP 60001124 A2 (07.01.85) and JP 63313727 A2 (21.12.88).
However, the disadvantage of incorporating a drug into a carrier system might be that an undesired change in the pharmacokinetics of the substance will be caused because the biodistribution is influenced by the biodistribution of the carrier, its RES activity and drug release from the carrier vehicle. Bogentoft et al. (in Folkers K., Littaru G. P., Yamagami T., (Eds.), Biomedical and Clinical Aspects of Coenzyme Q. Vol. 6. Elsevier 1991, pp. 215-224) observed that ubidecarenone accumulates in the RES organs when administered intravenously in a mixed micellar system or an emulsion vehicle, respectively. Furthermore, the solubility of the bioactive substance in the carrier is often too low to obtain therapeutic doses in acceptable volumes of the formulation. In addition, toxic side effects of the carrier particles by themselves have been discussed in the literature inter alia for parenteral lipid emulsions (Hajri T. et al., Biochim. Biophys. Acta 1047 (1990) 121-130; Connelly P. W. et al.; Biochim. Biophys. Acta 666 (1981) 80-89; Aviram M. et al., Biochem. Biophys. Res. Commun. 155 (1988) 709-713; Singh M. et al.; J. Parenter. Sci. Technol. 40 (1986) 34-40; Cotter R. et al., Am J. Clin. Nutr. 41 (1985) 994-1001; Untracht S., Biochim. Biophys. Acta711 (1982) 176-192).
From what is outlined above it is evident that ubidecarenone is a problematic substance with regard to pharmaceutical formulations of this drug. There are, however, by far more sparingly water soluble substances with similar formulation problems. The peroral bioavailability of these substances is poor due to the low aqueous solubility, and the intravenous administration is also problematic due to the lack of suitable intravenous formulations.